Electroacoustic transducers of the bilaminar flexural vibrating type

ABSTRACT

A button transducer comprises a metallic diaphragm concentrically bonded to a piezoelectric ceramic disc. A plurality of types of annular housings are provided for clamping and terminating the periphery of the diaphragm, each housing type having a central opening with a different diameter therein. The bonded diaphragm-disc assemblies are measured for resonant frequency, and the housing type is selected to tune the bonded assembly to have a predetermined resonant frequency.

v .v v. i, f i v [72] Inventor Frank N Cohasse [21] Appl. No. 859,677

[22] Filed Sept. 22, 1969 [45] Patented May 18, 1971 [73] Assignee Massa Division Dynamics Corporation of America Hingham, Mass.

[54] ELECTROACOUSTIC TRANSDUCERS OF THE BILAMINAR FLEXURAL VIBRATKNG TYPE 7 Qlaims, 5 Drawing Figs.

[52] US. Cl 310/8.8, 310/82, 310/8.5, 310/91 [51] Int. Cl l-IOlv 7/00 [50] Field of Search 310/82, 8.3, 8.5, 8.9, 8.8, 9.1, 9.4, 9.6; 29/2535; 340/10 [56] References Cited UNITED STATES PATENTS 3,331,970 7/1967 Dundon et al. 310/9.1

t., 4,17% aims 340/10 3,321,189 5/1967 Scarpa 259/144 3,401,275 9/1968 Curran et al.. 310/82 2,069,633 2/1937 Tripp 310/89 3,074,034 l/l963 Crownover 333/72 2,430,013 11/1947 Hausell 310/82 3,166,730 l/1965 Brown et al 340/10 Primary Examiner-Milt0n O. Hirshfield Assistant ExaminerB. A. Reynolds Altorney- Louis Bernat ABSTRACT: A button transducer comprises a metallic diaphragm concentrically bonded to a piezoelectric ceramic disc. A plurality of types of annular housings are provided for clamping and terminating the periphery of the diaphragm, each housing type having a central opening with a different diameter therein. The bonded diaphragm-disc assemblies are measured for resonant frequency, and the housing type is selected to tune the bonded assembly to have a predetermined resonant frequency.

Patented ay 18, 1971 3,578,995

/ N VE/VTOR.

FRANK MASSA MGM/47 Ely.

E11 .EECTROACOUSTIC TRANSDUCERS OF THE BELAMHJAR FLEXURAL VIBRATING TYPE This invention relates to new and improved electroacoustic transducers, and more particularly to low cost, mass produced transducers especially-although not exclusively-adapted for use in burglar alarm systems.

The problems of protecting areas from burglars or other intruders are solved by systems'using transmitting transducers which flood the protected area with sonic or ultrasonic energy. Any person or object moving in the protected area reflects this sonic energy in a manner which may be detected by a receiving transducer connected to drive a system which trips an alarm.

These references to transmitting and receiving transducers are made for convenience of expression only. The same functions may also be performed electronically by switching a single transducer between transmitting. and receiving modes of operation. Also, the reference here made to burglar alarm systems is purely by way of example since these systems represent a popular market for low cost, mass produced transducers having highly reliable precision characteristics. Those who are skilled in the art will readily perceive other uses for these transducers.

Accordingly, an object of the invention is to provide new and improved transducers of the described type. In this connection, an object is to provide low-cost transducers which have the precision heretofore available only in high priced transducers.

Another objectis to provide a transducer design that may initially have very wide production tolerances which may easily be reduced to acceptable tolerances after production in order to enable a final production which is equivalent to production with very tight initial tolerances.

Another object is to provide very small, unobtrusive transducers which do not attract an undue amount of attention.

Here an object is to provide self-checking transducers which send an alarm if subjected to tampering. Yet another object is to provide for enabling easy replacement of transducers for either maintenance or testing purposes.

Still another object of the invention is to provide a simple transducer which may be made quickly or easily on general purpose tooling. Here, an object is to reduce the capital cost of entry into the field.

Another object of this invention is to design a low-cost transducer for operating in the high audio or ultrasonic region.

Yet another object of this invention is to provide a low-cost production method for adjusting the resonant frequency of large quantities of transducers after they have been produced.

Still another object of this invention is to provide a low-cost transducer which may be universally used in a great variety of applications.

In keeping with an aspect of this invention, a small universal transducer is assembled as a button to be added to any other structure with which it is to be used. The transducer button is made very small in diameter so that the design is particularly useful in connection with ultrasonic devices used as proximity indicators or intrusion detectors where detection of the device by the intruder would foil the system. Then, the entire assembly including associated electrical components are built into a small cylindrical structure with a multiterminal plug connector at one end for simple plug-in installation in the field. Two of the terminals are internally connected to provide a continuity alarm circuit which is broken when the plug connector is removed from a socket.

Related transducers are disclosed in US. Pat. No. 3,128,532 and in a copending application, Ser. No. 866,785, filed Sept. 16, 1969, by Gilbert C. Barrow, both patent and application being assigned to the assignee of this invention. The disclosures of the patent and application may be read to obtain further information concerning the general subject matter.

The nature of this invention may be understood best from a study of the following specification when taken in connection with the attached drawings, in which:

FIG. 1 is a perspective view of an exemplary transducer, of the described type, attached to a canister housing having a multiterminal plug connection;

FIG. 2 is a plan view of the canister looking down upon the top of FIG. 1, along the line 2-2;

FIG. 3 is a sectional view of the transducer taken along the line 3-3 of FIG. 1;

FIG. 4 is a cross-sectional view taken along line 4-4 (FIG. 2) which illustrates the attachment of the transducer of FIG. 2 to a compact canister housing containing electronic components and a multiterminal electrical connecting plug for permitting convenient plug-in connections; and

FIG. 5 is a bottom plan view of FIG. 4.

This invention is concerned with improvements in the design of a small, inexpensive electroacoustic transducer 10 (FIG. 1) which employs a bilaminar circular disc assembly as the active vibratile diaphragm structure. This transducer is manufactured as a small button which may be press fitted or otherwise attached to any suitable housing, as it is here shown attached to the top of a canister housing 11. The canister may house any associated equipment, such as electrical components, for example. Another side of the canister may carry a multiterminal plug 12 for enabling the transducer and canister assembly to be plugged into a convenient outlet socket.

Other mounting structures may be used in lieu of the canisterplug combination, here shown. For example, the button transducer could be pressed into the cabinet or chassis of any associated electronic equipment.

The reference character 15 identifies a circular diaphragm preferably made of a rigid lightweight metal, such as aluminum. Attached to the central portion of the diaphragm 15 is a disc 16 of transducer material such as a polarized ceramic (for example, this may be barium titanate or lead zirconate titanate-both of which are well known in the art). The ceramic disc 16 has a metallized electrode surface on each face. The bonding material used to attach disc 16 to diaphragm 15 is preferably a rigid electrically conducting cement, such as an epoxy containing silver particles.

The diaphragm-transducer disc assembly 15, 16 is cemented into a peripheral recessed portion 17 of a massive ring or annular housing 18. Again, an electrically conductive epoxy or other suitable cement may be used to attach diaphragm 15 to ring 18; thus, the diaphragm disc 15 closes the top of the chamber 19. Opposite the diaphragm 15, the housing structure 18 contains another recessed chamber portion 21. These two chambers are separated by a closure disc 22 which may be made of insulation material.

A terminal lug 23 is attached to the disc 22 by means of a rivet 24. Another terminal lug 25 is attached to the housing 18 by a drive screw 28. The lug 23 makes electrical connection with one side of the piezoelectric disc 16, and the return circuit is via the housing, which makes electrical connection to the diaphragm 15 and the electrode surface of the piezoelectric disc 16.

The resonant frequency of the assembled vibratile structure varies with changes in the thickness of the diaphragm and ceramic. It also varies with the diameter of the opening through the ring clamp housing 18, which determines the clamped diameter of the disc. As a result, I have found that it is necessary to make the thickness of the diaphragm 15 less than one-fifteenth of its diameter. During vibration in the clamped disc mode of operation, only about one-third of the unclamped central area of the diaphragm develops a fibre stress of the same electrical polarity. Therefore, the area of the ceramic disc 16 should preferably be less than one-half of the total unclamped area of the diaphragm 15. I have also found that, to obtain efficient operation, the diameter of the ceramic disc 16 has to be more than 10 times the thickness of the ceramic.

This relationship between physical dimensions and resonant frequency affords an opportunity to reduce manufacturing costs. In greater detail, when manufacturing large quantities of transducers, I have found it economical to control the resonant frequency by following a sorting method. After assembling the diaphragms l5 and ceramic discs 16 and before mounting them on the ring clamp housings 18, the resonant frequencies of the free diaphragm-disc structures are measured. The structures are then separated into predetermined groups of different resonant frequencies. Experimentally, each such group has been found to require a housing 18 having an opening with a diameter D, which is required to adjust the resonant frequency of that group to have the desired value. Then, the housings with the larger openings D, are selected to be used with the higher frequency groups of diaphragm-disc structures. Housings with smaller openings D, are combined with diaphragm and ceramic disc assemblies displaying the lower resonant frequencies. By this simple procedure, the resonant frequencies of all the completed transducers are made the same, within acceptable manufacturing tolerances. This completes the structure (FIG. 3) which I sometimes call a button transducer.

FIGS. 4 and 5 illustrate one application in which a button transducer assembly is attached to a canister 11. In greater detail, the housing 18 of the transducer is applied to close one end of a cylindrical tube 11. The electronic components 30 and 31, required for the operation of the transducer, are mounted in the canister 11. Of course, these particular components are here shown by way of example only. Any suitable components may be mounted here.

The terminals 23 and 25, from the button transducer, are connected by wires to the components 30, 31. The open end of the tubular canister housing 11 is closed by a disc of insulating material 32 fitted with a number of connector pins 33, 34, 35, 36 and 37'which act as a plug-in connector. The pin 33 is slightly larger than the other pins in order to ensure proper orientation when the unit is plugged into the associated socket.

A volume control 40 is assembled in the center inside surface of the-insulated disc member 32. A countersunk slotted shaft 41 enables adjustment of the gain of the system.

A self-checking alarm system is provided to enable immediate detection if the unit is removed or tampered with in the field. In greater detail, a short-circuiting or continuity connection 42 is made between pins 36 and 37. This short circuit provides electrical continuity for a tamperproof circuit connected to the corresponding socket terminals. If the unit is removed, the shorting strip 42 no longer provides the continuity for the tamperproof circuit, and an alarm 45 may be given to indicate this fact.

After assembling all the components and making all the electrical connections inside the canister housing 11, a potting compound may be' poured into the canister in order to consolidate the structure.

Several specific and illustrative embodiments of the present invention have been shown and described. Of course, modifications may be made without departing from the true spirit and scope of the invention. Therefore, the appended claims are intended to cover all equivalents which fall within the true spirit and scope of the invention.

Iclaim:

1. An. electroacoustic transducer comprising a circular diaphragm having a diameter which is more than 15 times its thickness, acircular polarized piezoelectric ceramic disc having'a diameter which is more than 10 times its thickness, bonding means for holding said diaphragm and disc rigidly bound together in concentric face-to-face relationship, the area of said ceramic disc being no greater than one-half the area of said diaphragm, an annular housing structure having an opening at one end for locating and holding said diaphragm, rigid bonding means between the peripheral surface of said diaphragm and the surface in said opening in said annular housing, and electrical connection means attached to a circular face surface of said polarized ceramic disc.

2. The transducer of claim 1 further characterized in that said housing structure has a second opening at its other end and closure means comprising a circular insulating disc placed between said openingsto close said housing structure and seal off a compartment within which said ceramic disc is conminals attached thereto for establishing electrical connections from the outside of the assembled canister housing structure to the interior of said canister housing structure,

7. The transducer of claim 6 and means for short-circuiting a pair of said electrical connector terminals to provide a continuity alarm circuit. 

1. An electroacoustic transducer comprising a circular diaphragm having a diameter which is more than 15 times its thickness, a circular polarized piezoelectric ceramic disc having a diameter which is more than 10 times its thickness, bonding means for holding said diaphragm and disc rigidly bound together in concentric face-to-face relationship, the area of said ceramic disc being no greater than one-half the area of said diaphragm, an annular housing structure having an opening at one end for locating and holding said diaphragm, rigid bonding means between the peripheral surface of said diaphragm and the surface in said opening in said annular housing, and electrical connection means attached to a circular face surface of said polarized ceramic disc.
 2. The transducer of claim 1 further characterized in that said housing structure has a second opening at its other end and closure means comprising a circular insulating disc placed between said openings to close said housing structure and seal off a compartment within which said ceramic disc is contained.
 3. The transducer of claim 1 further characterized in that said diaphragm is a clamped metallic disc.
 4. The transducer of claim 3 further characterized in that said metal disc is aluminum.
 5. The transducer of claim 1 and a hollow tubular canister housing having said transducer attached to one end for effectively sealing said one end to close said canister housing, and closure means attached to seal the other end of said canister housing.
 6. The transducer of claim 5 wherein said closure is an electrical insulator with a plurality of electrical connector terminals attached thereto for establishing electrical connections from the outside of the assembled canister housing structure to the interior of said canister housing structure.
 7. The transducer of claim 6 and means for short-circuiting a pair of said electrical connector terminals to provide a continuity alarm circuit. 